Opposition MPs hammered the government Wednesday after CTV News reported that Sen. Pamela Wallin is being audited for hundreds of thousands of dollars in travel expenses over the past three years.

Wallin, who has denied there is anything wrong with her expense claims, is the fourth senator whose books have come under scrutiny in recent weeks.

Last week, it was announced that Deloitte will audit the housing expenses of Senators Mike Duffy, Mac Harb and Patrick Brazeau. All have denied any wrongdoing.

On Wednesday, NDP Leader Thomas Mulcair asked Prime Minister Stephen Harper about Wallin’s flights expenses.

“This is the taxpayers’ money that Sen. Wallin is using to travel around the country, to be the star during Conservative Party funding activities,” he said in French.

In response, Harper said he had looked at Wallin’s travel costs and they are “comparable to any parliamentarian.”

Expense records show Wallin, who was appointed to the upper chamber in 2009, has claimed $321,037 in “other travel” expenses since September 1, 2010. She also spent $29,423 on flights from Ottawa to her home province of Saskatchewan.

A source told CTV News that about 70 per cent of Wallin’s flights were to Toronto, where she also owns a home.

Sources say Wallin, a former broadcaster, offered to pay some of the money back after she was questioned by a Senate committee. But insiders say the committee refused and hired accounting firm Deloitte to investigate her expenses.

Wallin denied that in an email to CTV News.

“I certainly did willingly meet with a representative from Deloitte to review travel expenses and I answered all questions and have provided all the necessary information regarding claims,” she wrote.

“No offer of repayment was made or asked for. I spent 168 days in Saskatchewan last year and most of my travel was to my home province.”

The Senate called in outside auditors to examine Wallin’s flight expenses in early January.

Wallin defended her travel expenses in a radio interview Wednesday morning with Saskatoon’s 650 CKOM. She said many of her flights to Saskatchewan are not direct routes from Ottawa, so not listed as “other travel.”

“If I fly from Ottawa to Toronto and do an event -- say, go give a speech or go to a dinner or whatever it is I’m doing -- and if I sleep in Toronto, either in a hotel or a condo or whatever it is, then that flight the next morning -- where I fly from Toronto to Saskatoon or Toronto to Regina -- that flight then shows up in the categories that are made public on the Internet from the Senate as ‘other’ travel,” she said.

“When you look at the raw statistics, it goes, ‘Oh, she travels to other places. She doesn’t travel to her riding.’”

When asked about Wallin’s expenses on Parliament Hill Wednesday, Liberal Sen. Romeo Dallaire responded with surprise.

“I think it might be a little on the high side,” Dallaire told reporters. “But I wouldn’t know how you get to a figure like that.”

Last week, Brazeau was kicked out of the Conservative caucus and forced to take a leave of absence from the Senate after being charged with assault and sexual assault.

Even though he has lost his Senate perks and can’t attend any sessions in the upper chamber, Brazeau will continue to collect his $132,000 salary while the case is before the courts.

Brazeau’s books are also being looked at because he has stated his primary residence is his father’s home in Maniwaki, Que., even though he also rents a home in Gatineau, a short drive from Parliament Hill.

Duffy, a Conservative senator who is also a former broadcaster, has stated his primary residence is his home in Cavendish, P.E.I. But he was not on the voters’ list there in 2011, and instead cast a ballot in Ontario’s provincial election that year.

Duffy’s office also tried to expedite an application for a P.E.I. health card so the senator would have it ahead of a Jan. 31 deadline for all senators to submit proof of their primary residence.

Harb, a Liberal senator and a former MP, has also faced questions over expense claims for a secondary residence in Ottawa, even though he has lived in the capital for many years.

A $21,000 annual housing-and-meal allowance is offered to senators whose primary residence is more than 100 kilometres away from Ottawa. The Senate is now asking that all senators’ housing allowance claims be checked to determine their legitimacy.

Liberal Sen. David Smith urged caution before passing judgment on the senators until the audits and other investigations are complete.

“We want to be fair and give people due process,” Smith told CTV’s Power Play. “So let’s get the facts and then we deal with the facts. I don’t think it should be dealt with in a partisan manner. I think we want to be fair, be judicious and do the right thing.”

Smith called the current controversies “frustrating,” but defended the Senate’s work, saying it “does some things terrifically. I think our committees are very, very strong.”

He said most senators are committed to their work and the role they play, particularly when reviewing legislation.

“Sure when you’ve got just over 100 members there may be the odd incident relating to some people,” Smith said. “But when you look at the big picture I don’t think it’s going to cause somebody to try and start burning the place down.”

With a report from CTV’s Ottawa Bureau Chief Robert Fife